Asthma is an important national health problem, affecting 6 to 8% of the US population with 10% of all asthmatics having severe disease. The pathophysiology of asthma is heterogenous and involves multiple inflammatory cell types. Mast cells, known for their role in allergy and anaphylaxis, may lead to a deeper understanding of asthma severity but little is known regarding the levels or phenotype of intraepithelial and airway lumen mast cells (MCs) in asthma, particularly across disease severity. Objective: To determine the mast cell numbers, phenotype and local mast cell interactions present in the airway compartments of human subjects with a range of asthma severity compared to normal controls. Specific aims: 1) Determine the location, phenotype and function of airway mast cells in severe asthma, as compared to mild asthma and normal controls at the mRNA and protein level by evaluating the mast cell markers tryptase, chymase, carboxypeptidase 3, hematopoietic prostaglandin D2 synthase, and 5-lipoxygensae in epithelium, submucosal tissue and luminal (bronchoalveolar lavage cells) compartments. 2) Determine the presence and relative concentration of activation markers for mast cells including tryptase, histamine, and prostaglandin D2 in lavage fluid and sputum. 3) Develop the necessary technical skills to harvest mast cell populations in vitro from human cord blood in conjunction with the Boyce laboratory at Harvard Medical School/Brigham and Women's Hospital. 4) Determine whether factors produced by the epithelium, including TGFB1/2 and PGE2, support the growth and differentiation of epithelial mast cells in severe asthma. Research Design/Methods: MC phenotypic markers will be assessed using quantitative real time PCR, Western blot and immunohistochemistry techniques of fresh tissue samples. Activation will be assessed using EIA. In vitro mast cells derived from human cord blood will also be evaluated for MC phenotypic markers. Cultured MCs will be exposed in vitro to TGFB1/2, PGE2 and epithelial cell supernatants grown from asthmatic and normal subjects to further assess the interactions of the MC with the epithelium. Relevance to Public Health: Asthma is an important national health problem, affecting 6 to 8% of the US population with 10% of all asthmatics having severe disease. Patients with severe asthma (SA) often respond poorly to standard asthma treatment such corticosteroids. By identifying the distinct cell types involved in SA, such as the mast cell, new targeted therapies could improve patient quality of life.